1. Field of the Invention
The present invention relates to image processing devices, methods, and programs. In particular, the present invention relates to an image processing device, a method, and a program that can measure block noise strength.
2. Description of the Related Art
During decoding of encoded image data, noise may be generated in the image to be decoded.
For example, when image data is to be compressed based on a compression system such as an MPEG (moving picture experts group) compression system, an encoder divides the image data into rectangular blocks containing multiple pixels and performs DCT (discrete cosine transform) processing on each of the divided blocks.
Thus, when a decoder decodes the image data encoded based on the MPEG system, the decoded image data, in principle, gives rise to steps in pixel values between pixels at boundary portions of blocks and thus makes it easier for block noise to be generated.
A device for reducing or eliminating such block noise is realized by, typically, using a known block size (e.g., 8 pixels×8 pixels in the case of MPEG2) and smoothing the block noise by using a LPF (low pass filter) on block boundary positions.
Such a simple method, however, has problems in that information of an image is lost by blurring or the like or other block distortion occurs as a result of performing smoothing on only the block boundaries.
The method further has problems in that the amount and the strength of block noise vary greatly depending on the contents of an image, compression coding conditions (e.g., a bitrate and a quantization scale), and so on. Thus, the use of one type of block-noise reduction processing has problems in that no reduction effect can be obtained for a high block noise strength and a reduction in the amount of image information in the presence of a very small amount of block noise can cause an adverse effect.
In order to prevent those problems, various devices and methods have been proposed.
However, image signals subjected to scaling (conversion of a resolution/conversion of the number of pixels) by video output devices, such as DVDs (digital versatile players), may be input to television receivers, and thus, it is desired to be capable of handing a block size that is different from the block size (e.g., 8 pixels×8 pixels) specified by the MPEG standard.
That is, some of such players may have an function for performing scaling, and may thus convert even a recorded signal having a SD (standard definition) resolution, such as a 720 pixels×480 pixels, into a signal having a HD (high definition) resolution with 1920 pixels×1080 pixels for output.
For HD recording signals, signals having a resolution of 1440 pixels×1080 pixels also exist, and such signals may be converted into a HD resolution of 1920 pixels×1080 pixels for output.
As described above, depending on influences of the scaling and a difference in a system (e.g., for analog input video or digital input video), the quality of signals digitally decoded may vary greatly (e.g., distortion may be large or small).
The quality of a decoded image varies greatly ranging from a low quality to a high quality, depending on a difference in an encoding system, conditions (such as and a bitrate) for compression and encoding, or the type of image. In particular, for an image that involves high-speed motion and an image that contains complicated image information even in a still image, distortion tends to increase.
For example, Japanese Unexamined Patent Application Publication Nos. 2001-204029 and 2007-281542 disclose technologies for effectively performing noise reduction processing on a high-quality image to a low-quality image by estimating the quality of a compressed, decoded image and dynamically switching the intensity of the noise reduction processing.